Earlier quoted context omitted.
Any idea when exactly the PDF capture is performed? So many pages now render a minimal initial load, while the actual content is greatly delayed (usually Capturing the PDF at even the onload event (let alone the useless DOMContentLoaded event) is going to capture an incomplete picture in a huge number of situations. Frankly, a PDF capture at onload event is going to be utterly unusable. Is there a way to set a delay…
I don't think this feature is "utterly unusable". You can run your JS, wait for whatever events you want, and then pipe the resulting HTML to a --print-to-pdf command.
The primary reason to print a pdf would be to do so to any random 3rd party page. Sadly, you have no control over how 3rd party sites render/load their content. And --print-to-pdf likely captures the render of the page at some predetermined point, my guess being the "onload" event. Which, again, is not going to result in a fully rendered page for many sites that load in their primary content using AJAX.